First, as others have mentioned, the union did not unilaterally choose how the job specs at this plant were written.

But to answer your broader question, the reason unions sometimes point for seemingly narrow job specs is to prevent management picking and choosing titles in a way to keep pay and membership down.

Let's say that in this situation, you have 5 Server Janitors and 1 Sysadmin, with corresponding pay rates of course. Maybe the janitors have the skills to turn on the server, but the spec says only the sysadmin can. Whose fault is it that there's just one sysadmin? And if the job spec says anyone can turn the server on, why give anyone the sysadmin title?